To make informed prevention resource allocation decisions, public health decision makers require detailed information about the cost, cost-effectiveness, and potential impact of various strategies to prevent transmission of HIV. CAIR's Impact Science Core supports scientific research into the cost, economic efficiency, effectiveness, and real-world impact Of HIV/STD prevention interventions (including behavioral, biomedical, and structural intervention approaches) as well as intervention dissemination and technology transfer activities. The Core provides consultation to CAIR investigators regarding the feasibility of conducting economic efficiency or modeling studies of proposed, ongoing, or completed HIV prevention and dissemination/technology transfer interventions. Core Scientists identify appropriate economic evaluation or mathematical modeling techniques for assessing the effectiveness and cost-effectiveness of the intervention and collaborate with other CAIR scientists on the design of cost data collection instruments to support economic evaluation research and modeling studies. Core Scientists take a lead role in performing these analyses. The Core's support activities include prospective cost data elicitation during the course of an ongoing intervention trial; retrospective data collection after a trial is complete; mathematical modeling to quantify intervention effectiveness in epidemiologically-meaningful units (e.g., infections averted or expected number of secondary infections); and/or cost-effectiveness, cost-utility, or other economic analyses to determine the overall economic efficiency of HIV prevention interventions.